This invention relates to a method for manufacturing a semiconductor device employing a silicon substrate, more particularly a semiconductor device provided with at least two semiconductor elements of substantially the same type which vary from each other in electrical characteristics.
Heretofore, it has hardly been possible to form in the same step these two semiconductor elements, that is, as semiconductor elements transistors having substantially equal emitter top area and different electrical characteristics, with the current amplification factors .beta. of these transistors varying from each other, e.g., .beta.=200 and .beta.=100. In order to form such type of integrated circuit, therefore, the depths xj of the base regions of the two elements may be varied from each other in the base region forming step, or otherwise two kinds of impurities with different diffusion coefficients may be used in the emitter region forming step, whereby the manufacturing process will be complicated. Moreover, in the former case, the base region depth of a transistor with higher current amplification factor .beta. need be controlled to be shallower than that of one with lower current amplification factor .beta., so that the base regions of the two transistors must usually be formed in a separate base region forming step. In addition, if the base regions are formed in two steps, the impurities in the base region formed in the former step may be rediffused at the base diffusion in the latter process, so that it is very difficult to control the depth of the latter base region. Also in the latter case, the introduction of the different impurities requires two steps, and the control of the diffusion depth is difficult.